The chromosomal and sequence organization of muscle genes will be examined in Drosophila melanogaster. Previous studies of muscle cell differentiation have shown that the synthesis of at least 8 contractile proteins is activated simultaneously during myogenesis and the mRNAs for these proteins accumulate to high levels in differentiated muscle cells. In this study we propose to localize the nucleotide sequences coding for and adjacent to the contractile protein genes on polytene salivary gland chromosmes and to compare the sequence organization and homology of these genes in genomic DNA. Our objective is to examine whether these genes are closely linked on the salivary chromosomes and/or have homologous flanking or intervening nucleotide sequences which may play a role in coordinating the expression of these genes during myogenesis. Double-stranded cDNA copies of Drosophila muscle mRNAs will be cloned using the plasmid pBR322 as a vector and clones for muscle-specific mRNAs selected by colony hybridization. Clones carrying sequences complementary to mRNAs will be identified by analysis of the cell-free translation products synthesized by mRNAs which form hybrids with individual cloned cDNA sequences. Identified cDNA sequences will then be used as hybridization probes to localize contractile protein gene coding sequences or families of sequences on polytene salivary chromosomes and to screen a lambda phage library of Drosophila DNA to isolate genomic DNA carrying the contractile protein genes sequences. Genomic DNA sequences of the different contractile protein genes will then be compared by restriction enzyme mapping and hybridization, R-loop and heteroduplex to loci on salivary chromosomes. These studies will establish the extent of linkage and sequence homology between these genes and adjacent sequences on the salivary chromosome map.